[ 18 F]FDG and [ 11 C]PK11195 PET imaging in the evaluation of brown adipose tissue - effects of cold and pharmacological stimuli and their association with crotamine intake in a male mouse model.

UNLABELLED: This study aimed to evaluate the role of positron emission tomography (PET) with [11 C]PK11195 and [18 F]FDG in the characterization of brown adipose tissue (BAT).

METHODS: Male C57BL/6 mice were studied with the glucose analogue [18 F]FDG (n = 21) and the TSPO mitochondrial tracer [11 C]PK11195 (n = 28), without stimulus and after cold (6-9 °C) or beta-agonist (CL316243) stimuli. PET studies were performed at baseline and after 21 days of daily treatment with crotamine, which is a peptide described to induce adipocyte tissue browning and to increase BAT metabolism. Tracer uptake (SUVmax) was measured in the interscapular BAT and translocator protein 18 kDa (TSPO) expression was evaluated by immunohistochemistry.

RESULTS: The cold stimulus increased [18 F]FDG uptake compared to no-stimulus (5.21 ± 1.05 vs. 2.03 ± 0.21, p < 0.0001) and to beta-agonist stimulus (2.65 ± 0.39, p = 0.0003). After 21 days of treatment with crotamine, there was no significant difference in the [18 F]FDG uptake compared to the baseline in the no-stimulus group and in the cold-stimulus group, with a significant increase in uptake after CL stimulus (baseline: 2.65 ± 0.39; 21 days crotamine: 4.77 ± 0.81, p = 0.0003). Evaluation of [11 C]PK11195 at baseline shows that CL stimulus increases the BAT uptake compared to no-stimulus (4.47 ± 0.66 vs. 3.36 ± 0.68, p = 0.014). After 21 days of treatment with crotamine, there was no significant difference in the [11 C]PK11195 uptake compared to the baseline in the no-stimulus group (2.94 ± 0.58, p = 0.7864) and also after CL stimulus (3.55 ± 0.79, p = 0.085). TSPO expression correlated with [11 C]PK11195 uptake (r = 0.83, p = 0.018) but not with [18 F]FDG uptake (r = 0.40, p = 0.516).

CONCLUSIONS: [11 C]PK11195 allowed the identification of BAT under thermoneutral conditions or after beta3-adrenergic stimulation in a direct correlation with TSPO expression. The beta-adrenergic stimulus, despite presenting a lower intensity of glycolytic activation compared to cold at baseline, allowed the observation of an increase in BAT uptake of [18 F]FDG after 21 days of crotamine administration. Although some limitations were observed for the metabolic changes induced by crotamine, this study reinforced the potential of using [11 C]PK11195 and/or [18 F]FDG-PET to monitor the activation of BAT.